Input/output circuit and method

ABSTRACT

A circuit includes a first power node configured to carry a first voltage having a first voltage level, a second power node configured to carry a second voltage having a second voltage level, an output node, and first and second cascode transistors coupled between the first power node and the output node and to each other at a node. A bias circuit uses the first and second cascode transistors to generate an output signal at the output node that transitions between the first voltage level and a third voltage level, and a delay circuit generates a transition in a first signal from one of the first or second voltage levels to the other of the first or second voltage levels, the transition having a time delay based on the output signal. A contending transistor couples the node to the second power node responsive to the first signal.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No. 16/206,577, filed Nov. 30, 2018, now U.S. Pat. No. 10,686,434, issued Jun. 16, 2020, which is a continuation of U.S. application Ser. No. 15/244,152, filed Aug. 23, 2016, now U.S. Pat. No. 10,187,046, issued Jan. 22, 2019, which is a continuation of U.S. application Ser. No. 14/630,934, filed Feb. 25, 2015, now U.S. Pat. No. 9,450,573, issued Sep. 20, 2016, each of which is incorporated herein by reference in its entirety.

RELATED APPLICATION

The instant application is related to U.S. application Ser. No. 14/189,653, filed Feb. 25, 2014, now U.S. Pat. No. 9,214,933, issued Dec. 15, 2015. The entire contents of the above-referenced application are incorporated by reference herein.

BACKGROUND

As semiconductor technology develops, an integrated circuit sometimes has a signal operating at a voltage swing lower than that of a signal suitable for an external circuit, such as another integrated circuit or one or more discrete electrical components. An input/output (I/O) circuit is often used in the integrated circuit in order to convert the low voltage swing signal from the integrated circuit to a high voltage swing signal recognizable by the external circuit. In some applications, the integrated circuit includes low voltage transistors and high voltage transistors. Low voltage transistors are sometimes also referred to as core (or thin-gate) transistors and are configured to handle the low voltage swing signal. High voltage transistors are sometimes also referred to as I/O (or thick-gate) devices and are configured to handle the large voltage swing signal. Core transistors are designed to be sufficiently large to handle the low voltage swing signal, but are usually not large enough to handle the large voltage swing signal. On the other hand, compared with low-voltage transistors, I/O transistors are usually larger and occupy a larger die space.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a circuit diagram of an example I/O circuit in accordance with some embodiments.

FIGS. 2A and 2B are timing diagrams of voltage signals at various nodes of the I/O circuit of FIG. 1 in accordance with some embodiments.

FIG. 3A is a circuit diagram of an example control circuit usable in the I/O circuit of FIG. 1 in accordance with some embodiments.

FIG. 3B is a timing diagram of the control circuit of FIG. 3A in accordance with some embodiments.

FIG. 4A is a circuit diagram of an example control circuit usable in the I/O circuit of FIG. 1 in accordance with some embodiments.

FIG. 4B is a timing diagram of the control circuit of FIG. 4A in accordance with some embodiments.

FIG. 5A is a circuit diagram of an example control circuit usable in the I/O circuit of FIG. 1 in accordance with some embodiments.

FIG. 5B is a timing diagram the control circuit of FIG. 5A in accordance with some embodiments.

FIG. 6 is a circuit diagram of an example I/O circuit in accordance with some embodiments.

FIG. 7 is a circuit diagram of an example I/O circuit in accordance with some embodiments.

FIG. 8 is a flow chart of a method of operating an I/O circuit in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

In accordance with one or more embodiments, an I/O circuit includes pull-up cascode driver transistors, pull-down cascode driver transistors, and one or more contending circuits. The contending circuits are usable to prevent a voltage stress applied to the pull-up cascode driver transistors or the pull-down cascode driver transistors when the I/O circuit is used to drive a large load.

FIG. 1 is a circuit diagram of an example I/O circuit 100 in accordance with some embodiments. I/O circuit 100 includes an input node 102 and an output node 104. Circuit 100 is configured to receive an input signal V_(IN) at input node 102 and to generate an output signal V_(OUT) at output node 104. Input signal V_(IN) has a voltage level ranging from a zero reference level (i.e., 0 volt for circuit 100) to a predetermined voltage level V_(DD). Output signal V_(OUT) has a voltage level ranging from the zero reference level to a predetermined voltage level K·V_(DD). K is a positive ratio greater than 1. V_(DD) is a predetermined, non-zero value above the zero reference level. In some embodiments, V_(DD) ranges from 0.65 Volts (V) to 1.20 V. Input signal V_(IN) is a logic signal usable to indicate a logic low value when input signal V_(IN) is set at the zero reference level and to indicate a logic high value when input signal V_(IN) is set at V_(DD). Output signal V_(OUT) is a logic signal usable to indicate a logic high value (K·V_(DD)) when input signal V_(IN) is set at the zero reference level and to indicate a logic low value (the zero reference level) when input signal V_(IN) is set at V_(DD). In FIG. 1, output signal V_(OUT) is logically complementary to input signal V_(IN), in some embodiments, and time-shifted by a time delay attributable to the operation of I/O circuit 100.

I/O circuit 100 includes a first power node 112, a second power node 114, a third power node 116, and a fourth power node 118. I/O circuit 100 also includes a first pull-up driver transistor 122 and a second pull-up driver transistor 124 coupled in series between power node 112 and output node 104; a first pull-down driver transistor 132 and a second pull-down driver transistor 134 coupled in series between power node 114 and output node 104; a biasing circuit 140 configured to control the pull-up driver transistors 122 and 124 and the pull-down driver transistors 132 and 134 based on input signal V_(IN); and contending circuits 150 and 160 configured to contend with driver transistors 122 or 132 under certain operation conditions. Output node 104 of I/O circuit 100 is coupled with an internal load 182, including at least an I/O pad and an electrostatic discharge (ESD) protection circuit, and an external load 184. Internal load 182 and external load 184 are collectively represented by a capacitive load 180 in this application.

Driver transistor 122 is a P-type transistor having a source coupled with power node 112. Driver transistor 124 is a P-type transistor having a source coupled with a drain of driver transistor 122 and a drain coupled with output node 104. The node corresponding to the source of the driver transistor 124 and the drain of the driver transistor 122 is labeled as node 126. Driver transistors 122 and 124 are also known as cascode transistors. Driver transistor 132 is an N-type transistor having a source coupled with power node 114. Driver transistor 134 is an N-type transistor having a source coupled with a drain of driver transistor 132 and a drain coupled with output node 104. The node corresponding to the source of the driver transistor 134 and the drain of the driver transistor 132 is labeled as node 136. Driver transistors 132 and 134 are also known as cascode transistors.

Biasing circuit 140 is coupled with input node 102 and gates of driver transistors 122, 124, 132, and 134. Biasing circuit 140 is configured to turn off driver transistors 122 and 124 and turn on driver transistors 132 and 134 responsive to input signal V_(IN) being at the logical high value; and to turn on driver transistors 122 and 124 and turn off driver transistors 132 and 134 responsive to input signal V_(IN) being at the logical low value. In some embodiments when K is 2, the voltage level at the gates of driver transistors 124 and 134 are set at V_(DD). Biasing circuit 140 also includes a level shifter 142 and a delay unit 144 between input node 102 and the gate of driver transistor 122, and a delay unit 146 between input node 102 and the gate of driver transistor 132. Level shifter 142 is configured to generate an intermediate signal V_(INT) by up-shifting input signal V_(IN) by V_(DD). Delay unit 144 is configured to generate the signal to be fed to the gate of driver transistor 122 by delaying the intermediate signal V_(INT). Delay unit 146 is configured to generate the signal to be fed to the gate of driver transistor 132 by delaying the input signal V_(IN).

Delay units 144 and 146 are usable to synchronize the timing of transitions of the bias voltage for the gates of driver transistors 122 and 132. In some embodiments, the delay periods of delay units 144 and 146 are tunable according to one or more control signals, either in an analog or digital format. In some embodiments, the delay periods of delay units 144 and 146 are predetermined and fixed when delay units 144 and 146 are fabricated.

In some applications, the capacitive load 180 causes the output signal V_(OUT) to have a slew rate slower than the slew rate of the signal at the gate of driver transistor 122 or the slew rate of the signal at the gate of driver transistor 132. Therefore, in some occasions, when driver transistor 122 is turned on, a voltage difference between node 126 and output signal V_(OUT) is greater than V_(DD). Also, in some occasions, when driver transistor 132 is turned on, a voltage difference between node 136 and output signal V_(OUT) is greater than V_(DD). However, in some applications, driver transistors 124 and 134 are not designed to repetitively withstand a source-to-drain voltage greater than V_(DD). The issue caused by voltage over-stress to driver transistors 124 and 134 are also known as a “hot-carrier-injection” issue. In some applications, the “hot-carrier-injection” issue causes degradation of the reliability of I/O circuit 100 when the slew rate of the output signal V_(OUT) is greater than 10 ns.

Contending circuit 150 is configured to pull the drain of driver transistor 122 toward a third voltage level at third power node 116 during a time period after the driver transistor 122 is set to pull the drain of the driver transistor 122 toward the first voltage level at third power node 116. The time period is determined based on a slew rate of output signal V_(OUT), and the third voltage level is less than the first voltage level. As a result, a source-to-drain voltage of driver transistor 124 when driver transistor 122 is turned on is mitigated by contending circuit 150. In some embodiments, a voltage difference between the first voltage level and the third voltage level is equal to or less than V_(DD). In some embodiments, the first voltage level is 2·V_(DD), and the third voltage level is V_(DD).

Contending circuit 150 includes a slew rate detection circuit 152 configured to generate a feedback signal V_(SRP) based on a rising edge of output signal V_(OUT), a control circuit 154 configured to generate a control signal V_(CDP) based on a signal V_(INP) at the gate of driver transistor 122 and feedback signal V_(SRP), and a contending transistor 156 between the drain of the driver transistor 122 (i.e., node 126) and third voltage node 116. Contending transistor is an N-type transistor in the embodiment depicted in FIG. 1 and has a gate configured to receive the control signal V_(CDP). In some embodiments, contending transistor 156 has a driving capability equal to or less than driver transistor 122.

Contending circuit 160 includes a slew rate detection circuit 162 configured to generate a feedback signal V_(SRN) based on a falling edge of output signal V_(OUT), a control circuit 164 configured to generate a control signal V_(CDN) based on a signal V_(INN) at the gate of driver transistor 132 and feedback signal V_(SRN), and a contending transistor 166 between the drain of the driver transistor 132 (i.e., node 136) and fourth voltage node 118. Contending transistor is a P-type transistor in the embodiment depicted in FIG. 1 and has a gate configured to receive the control signal V_(CDN). In some embodiments, contending transistor 166 has a driving capability equal to or less than driver transistor 132.

In some embodiments, one of the contending circuit 150 or contending circuit 160 is omitted. In some embodiments, slew rate detection circuit 152 and slew rate detection circuit 162 are implemented as an integrated slew rate detection circuit.

FIG. 2A is a timing diagram of voltage signals at various nodes of the I/O circuit 100 when input signal V_(IN) transitions from the logical low value to the logical high value in accordance with some embodiments. FIG. 2A is illustrated in conjunction with some components depicted in FIG. 1 and the reference numbers assigned thereof.

In FIG. 2A, curve 212 represents signal V_(INN) at the gate of driver transistor 132; curve 214 represents signal V_(CDN) at the gate of contending transistor 166; curve 222 represents a signal V_(DN) at drain of driver transistor 132; and curve 224 represents signal V_(OUT) at output node 104. Curve 232 represents the voltage difference between signal V_(OUT) and signal V_(DN).

Moreover, curve 222′ represents a signal V_(DN) at drain of driver transistor 132 in an alternative I/O circuit having a configuration without contending circuit 160; curve 224′ represents signal V_(OUT) at output node 104 in such alternative I/O circuit; and curve 232′ represents the voltage difference between signal V_(OUT) and signal V_(DN) in this alternative I/O circuit.

At time T₂₁₁, biasing circuit causes signal V_(INN) (curve 212) to transit from the zero reference level to voltage V_(DD) (which is set to be 1.8 V in this embodiment). At time T₂₁₂, the voltage level of signal V_(INN) (curve 212) starts to turn on driver transistor 132. As a result, output signal V_(OUT) (curve 224) starts to transition from 2·V_(DD) (which is set to be 3.6 V in this embodiment) to the zero reference level. A slew rate of the falling edge of output signal V_(OUT) depends on the driving capability of driver transistors 132 and 134 and the capacitive load 180. Based on a slew rate of the falling edge of output signal V_(OUT) (curve 224) detected by slew rate detection circuit 162, control circuit 164 keeps signal V_(CDN) (curve 214) at the zero reference level for the time being. Therefore, contending transistor 166 is turned on responsive to signal V_(CDN), and signal V_(DN) (curve 222) remains at a voltage level close to V_(DD) as a result of current contention between contending transistor 166 and driver transistor 132.

At time T₂₁₃, the voltage level of signal V_(INN) (curve 212) reaches V_(DD). At time T₂₁₄, based on a slew rate of the falling edge of output signal V_(OUT) (curve 224) detected by slew rate detection circuit 162, control circuit 164 causes signal V_(CDN) (curve 214) to transition from the zero reference level to V_(DD). The rising edge of signal V_(CDN) is generated by delaying the rising edge of signal V_(INN) for a delay period T_(D1), where the delay period T_(D1) is determined by control circuit 164 based on the feedback signal V_(SRN) from slew rate detection circuit 162. The voltage level of signal V_(CDN) (curve 214) starts to turn off contending transistor 166. As contending transistor 166 is being gradually turned off by signal V_(CDN), signal V_(DN) (curve 222) starts to transition toward the zero reference level by driver transistor 132.

At time T₂₁₅, the voltage level of signal V_(CDN) (curve 212) reaches V_(DD), contending transistor 166 is fully turned off, and the voltage level of signal V_(DN) (curve 222) has reached about 0.1·V_(DD) above the zero reference level. At time T₂₁₆, the voltage level of output signal V_(OUT) (curve 224) reaches the zero reference level.

As illustrated by curve 232, a voltage difference between output signal V_(OUT) and signal V_(DN) is controlled to be less than or equal to V_(DD).

In comparison with the embodiment described in conjunction with curves 212, 214, 222, 224 and 232, the operation of the alternative I/O circuit having a configuration without contending circuit 160 is illustrated in conjunction with curves 212, 222′, 224′, and 232′. Responsive to the rising edge of signal V_(INN) (curve 212), signal V_(DN) (curve 222′) begins to transition from V_(DD) to the zero reference level at time T₂₁₂. The voltage level of signal V_(DN) (curve 222′) reaches about 0.1·V_(DD) above the zero reference level at time T₂₁₃. Output signal V_(OUT) (curve 224′) begins to transition from 2·V_(DD) to the zero reference level at time T₂₁₂ and reaches the zero reference level right after time T₂₁₅. In the alternative I/O circuit, a voltage difference between output signal V_(OUT) and signal V_(DN) exceeds V_(DD) at time T₂₁₂ and is not pulled back to be under V_(DD) until about the half way between time T₂₁₃ and time T₂₁₄.

Comparing curves 224 and 224′, it takes longer for the output signal V_(OUT) of the I/O circuit with contending circuit 160 to transition from 2·V_(DD) to the zero reference level responsive to the rising edge of signal V_(INN) than the alternative I/O circuit without contending circuit 160. In some embodiments, contending circuit 160 is configured to keep the responsive speed of output signal V_(OUT) within a predetermined design specification. However, comparing curves 232 and 232′, the I/O circuit with contending circuit 160 has a much lower risk to stress or damage driver transistor 134 than the alternative I/O circuit without contending circuit 160.

FIG. 2B is a timing diagram of voltage signals at various nodes of the I/O circuit 100 when input signal V_(IN) transitions from the logical high value to the logical low value in accordance with some embodiments. FIG. 2B is illustrated in conjunction with some components depicted in FIG. 1 and the reference numbers assigned thereof.

In FIG. 2B, curve 252 represents signal V_(INP) at the gate of driver transistor 122; curve 254 represents signal V_(CDP) at gate of contending transistor 156; curve 262 represents a signal V_(DP) at drain of driver transistor 122; and curve 264 represents signal V_(OUT) at output node 104. Curve 272 represents the voltage difference between signal V_(DP) and signal V_(OUT).

Moreover, curve 262′ represents a signal V_(DP) at drain of driver transistor 122 in an alternative I/O circuit having a configuration without contending circuit 150; curve 264′ represents signal V_(OUT) at output node 104 in such alternative I/O circuit; and curve 272′ represents the voltage difference between signal V_(DP) and signal V_(OUT) in this alternative I/O circuit.

At time T₂₂₁, biasing circuit causes signal V_(INP) (curve 252) to transition from 2·V_(DD) to V_(DD). At time T₂₂₂, the voltage level of signal V_(INP) (curve 252) starts to turn on driver transistor 122 and output signal V_(OUT) (curve 254) thus starts to transition from the zero reference level to 2·V_(DD). A slew rate of the rising edge of output signal V_(OUT) depends on the driving capability of driver transistors 122 and 124 and the capacitive load 180. Based on a slew rate of the rising edge of output signal V_(OUT) (curve 264) detected by slew rate detection circuit 152, control circuit 154 keeps signal V_(CDP) (curve 254) at 2·V_(DD) for the time being. Therefore, contending transistor 156 is turned on responsive to signal V_(CDP), and signal V_(DP) (curve 262) remains at a voltage level close to V_(DD) as a result of current contention between contending transistor 156 and driver transistor 122.

At time T₂₂₃, the voltage level of signal V_(INP) (curve 252) reaches V_(DD). At time T₂₂₄, based on a slew rate of the rising edge of output signal V_(OUT) (curve 264) detected by slew rate detection circuit 152, control circuit 154 causes signal V_(CDP) (curve 254) to transition from 2·V_(DD) to V_(DD). The falling edge of signal V_(CDP) is generated by delaying the falling edge of signal V_(INP) for a delay period T_(D2), where the delay period T_(D2) is determined by control circuit 154 based on the feedback signal V_(SRP) from slew rate detection circuit 152. The voltage level of signal V_(CDP) (curve 254) starts to turn off contending transistor 156. As contending transistor 156 is being gradually turned off by signal V_(CDP), signal V_(DP) (curve 262) starts to transition toward 2·V_(DD) by driver transistor 122.

At time T₂₂₅, the voltage level of signal V_(CDP) (curve 254) reaches V_(DD), contending transistor 156 is fully turned off, and the voltage level of signal V_(DP) (curve 262) has reached about 0.1·V_(DD) below 2·V_(DD). At time T₂₂₆, the voltage level of output signal V_(OUT) (curve 264) reaches 2·V_(DD).

As illustrated by curve 272, a voltage difference between signal V_(DP) and output signal V_(OUT) is controlled to be no greater than V_(DD).

Similar to the comparison between an I/O circuit with contending circuit 160 and an I/O circuit without contending circuit 160 depicted in FIG. 2A, responsive to the falling edge of signal V_(INP) (curve 252), signal V_(DP) (curve 262′) begins to transition from V_(DD) to 2·V_(DD) at time T₂₂₂. The voltage level of signal V_(DP) (curve 262′) reaches about 0.1·V_(DD) below 2·V_(DD) at time T₂₂₃. Output signal V_(OUT) (curve 264′) begins to transition from the zero reference level to 2·V_(DD) at time T₂₂₂ and reaches 2·V_(DD) right after time T₂₂₅. In the alternative I/O circuit, a voltage difference between signal V_(DP) and output signal \Tour exceeds V_(DD) at time T₂₂₂ and is not pulled back to be under V_(DD) until about the half way between time T₂₂₃ and time T₂₂₄.

FIG. 3A is a circuit diagram of an example control circuit 300 usable as control circuit 164 in the I/O circuit 100 of FIG. 1 in accordance with some embodiments. Components that are the same or similar to those depicted in FIG. 1 are given the same reference numbers, and detailed description thereof is thus omitted.

Control circuit 300 includes a first power node 302 and a second power node 304. In some embodiments, power node 302 is configured to receive a voltage having the voltage level the same as that of power node 118 (FIG. 1, V_(DD)), and power node 304 is configured to receive a voltage having the voltage level the same as that of power node 114 (the zero reference level). Components of control circuit 300 are operated in a power domain defined by power node 302 and power node 304.

Control circuit 300 includes an adjustable delay circuit 312 and an AND gate 314. Adjustable delay circuit 312 has an input node 312 a configured to receive signal V_(INN) and an output node 312 b. Adjustable delay circuit 312 is configured to generate a delayed signal V_(INN)′ by delaying the signal V_(INN) based on the feedback signal V_(SRN). In some embodiments, when the output signal V_(OUT) has a slower slew rate, adjustable delay circuit 312 is set to have a greater delay.

AND gate 314 having a first input 314 a, a second input 314 b, and an output 314 c. First input 314 a of the AND gate 314 is configured to receive the signal V_(INN), and second input 314 b of the AND gate 314 is configured to receive the delayed signal V_(INN)′ from the adjustable delay circuit 312. Output 314 c of the AND gate 314 is coupled with, and configured to provide control signal V_(CDN) to, the gate of the contention transistor 166.

FIG. 3B is a timing diagram of the control circuit of FIG. 3A in accordance with some embodiments. Curve 322 represents signal V_(INN); curve 324 represents signal V_(INN)′; and curve 326 represents signal V_(CDN). As depicted in FIGS. 3A and 3B, delay circuit 312 generates the rising edge of delayed signal V_(INN)′ by delaying the corresponding rising edge of signal V_(INN) for a time delay T_(D1). Time delay T_(D1) is set based on the feedback signal V_(SRN). Also, AND gate 314 generates a rising edge of signal V_(CDN) by passing though the corresponding rising edge of signal V_(INN)′ and generates a falling edge of signal V_(CDN) by passing though the corresponding falling edge of signal V_(INN). By passing along the falling edge of signal V_(INN) to signal V_(CDN), the voltage level at node 136 (i.e., signal V_(DN)) is at least controlled by either driver transistor 132 or contention transistor 166 when signal V_(IN) transitions from logical high value to logical low value.

FIG. 4A is a circuit diagram of another example control circuit 400 usable as control circuit 154 in the I/O circuit of FIG. 1 in accordance with some embodiments. Components that are the same or similar to those depicted in FIG. 1 are given the same reference numbers, and detailed description thereof is thus omitted.

Control circuit 400 includes a first power node 402 and a second power node 404. In some embodiments, power node 402 is configured to receive a voltage having the voltage level the same as that of power node 112 (FIG. 1, 2·V_(DD)), and power node 404 is configured to receive a voltage having the voltage level the same as that of power node 116 (V_(DD)). Components of control circuit 400 are operated in a power domain defined by power node 402 and power node 404.

Control circuit 400 includes an adjustable delay circuit 412 and an OR gate 414. Adjustable delay circuit 412 has an input node 412 a configured to receive signal V_(INP) and an output node 412 b. Adjustable delay circuit 412 is configured to generate a delayed signal V_(INP)′ by delaying the signal V_(INP) based on the feedback signal V_(SRP). In some embodiments, when the output signal V_(OUT) has a slower slew rate, adjustable delay circuit 412 is set to have a greater delay.

OR gate 414 having a first input 414 a, a second input 414 b, and an output 414 c. First input 414 a of the OR gate 414 is configured to receive the signal V_(INP), and second input 414 b of the OR gate 414 is configured to receive the delayed signal V_(INP)′ from the adjustable delay circuit 412. Output 414 c of the OR gate 414 is coupled with, and configured to provide control signal V_(CDP) to, the gate of the contending transistor 156.

FIG. 4B is a timing diagram the control circuit of FIG. 4A in accordance with some embodiments. Curve 422 represents signal V_(INP); curve 424 represents signal NP′; and curve 426 represents signal V_(CDP). As depicted in FIGS. 4A and 4B, delay circuit 412 generates the falling edge of delayed signal V_(INP)′ by delaying the corresponding falling edge of signal V_(INP) for a time delay T_(D2). Time delay T_(D2) is set based on the feedback signal V_(SRP). Also, OR gate 414 generates a falling edge of signal V_(CDP) by passing though the corresponding falling edge of signal V_(INP)′ and generates a rising edge of signal V_(CDP) by passing though the corresponding rising edge of signal V_(INP). By passing along the rising edge of signal V_(INP) to signal V_(CDP), the voltage level at node 126 (i.e., signal V_(DP)) is at least controlled by either driver transistor 122 or contention transistor 156 when signal V_(IN) transitions from logical low value to logical high value.

FIG. 5A is a circuit diagram of another example control circuit 500 usable as control circuit 154 or control circuit 164 in the I/O circuit of FIG. 1 in accordance with some embodiments. Components that are the same or similar to those depicted in FIG. 1 are given the same reference numbers, and detailed description thereof is thus omitted.

Control circuit 500 includes a first power node 502 and a second power node 504. Components of control circuit 500 are operated in a power domain defined by power node 502 and power node 504.

In some embodiments when control circuit 500 is used as control circuit 154, power node 502 is configured to receive a voltage having the voltage level the same as that of power node 112 (FIG. 1, 2·V_(DD)), and power node 504 is configured to receive a voltage having the voltage level the same as that of power node 116 (V_(DD)). In some embodiments when control circuit 500 is used as control circuit 164, power node 502 is configured to receive a voltage having the voltage level the same as that of power node 118 (FIG. 1, V_(DD)), and power node 504 is configured to receive a voltage having the voltage level the same as that of power node 114 (the zero reference level).

Control circuit 500 includes an adjustable delay circuit configured to generate control signal V_(CDP) or V_(CDN) by delaying the corresponding signal V_(INP) or V_(INN) based on the feedback signal V_(SRP) or V_(SRN). In some embodiments, when the output signal V_(OUT) has a slower slew rate, adjustable delay circuit 512 is set to have a greater delay.

FIG. 5B is a timing diagram the control circuit of FIG. 5A in accordance with some embodiments. Curve 522 represents signal V_(INN) and curve 524 represents signal V_(CDN) when control circuit 500 is used as control circuit 164. Curve 526 represents signal V_(INP) and curve 528 represents signal V_(CDP) when control circuit 500 is used as control circuit 154.

As depicted in FIGS. 5A and 5B, when control circuit 500 is used as control circuit 164, control circuit 500 generates the rising edge of signal V_(CDN) by delaying the corresponding rising edge of signal V_(INN) for a time delay T_(D1) and generates the falling edge of signal V_(CDN) by delaying the corresponding falling edge of signal V_(INN) for a time delay T_(D3). Time delays T_(D1) and T_(D3) are set based on the feedback signal V_(SRN). Compared with using control circuit 300, both driver transistor 132 and contention transistor 166 are turned off during the time period T_(D3), and thus node 136 is temporarily set to be electrically floating.

Moreover, when control circuit 500 is used as control circuit 154, control circuit 500 generates the falling edge of signal V_(CDP) by delaying the corresponding falling edge of signal V_(INP) for a time delay T_(D2) and generates the rising edge of signal V_(CDP) by delaying the corresponding rising edge of signal V_(INP) for a time delay T_(D4). Time delays T_(D2) and T_(D4) are set based on the feedback signal V_(SRP). Compared with using control circuit 400, both driver transistor 122 and contention transistor 156 are turned off during the time period T_(D4), and thus node 126 is temporarily set to be electrically floating.

FIG. 6 is a circuit diagram of another example I/O circuit 600 in accordance with some embodiments. Components that are the same or similar to those depicted in FIG. 1 are given the same reference numbers, and detailed description thereof is thus omitted.

Compared with I/O circuit 100, circuit 600 further includes a P-type driver transistor 622 between output node 104 and driver transistor 124; an N-type driver transistor 632 between output node 104 and driver transistor 134; and a biasing circuit 640 replacing biasing circuit 140. Output signal V_(OUT) has a voltage level ranging from the zero reference level to a predetermined voltage level 3·V_(DD). Power node 112 is configured to receive a voltage having a voltage level of 3·V_(DD). Power node 116 is configured to receive a voltage having a voltage level of 2·V_(DD).

Biasing circuit 640 is coupled with input node 102 and gates of driver transistors 122, 124, 622, 132, 134, and 632. Biasing circuit 640 is configured to turn off driver transistors 122, 124, and 622 and turn on driver transistors 132, 134, and 632 responsive to input signal V_(IN) being at the logical high value; and to turn on driver transistors 122, 124, and 622 and turn off driver transistors 132, 134, and 632 responsive to input signal V_(IN) being at the logical low value. In some embodiments, biasing circuit 640 outputs an up-shifted signal ranging from 2·V_(DD) to 3·V_(DD) to the gate of driver transistor 122; an up-shifted, inverted, signal ranging from 2·V_(DD) to V_(DD) to the gates of driver transistor 622 and 632; and a signal ranging from 0 to V_(DD) to the gate of driver transistor 132. In some embodiments, biasing circuit 640 outputs 2·V_(DD) to the gate of driver transistor 124 and V_(DD) to the gate of driver transistor 134.

The operation of contending circuits 150 and 160 in I/O circuit 600 are similar to the operation of contending circuits 150 and 160 in I/O circuit 100, and detailed description is thus omitted.

FIG. 7 is a circuit diagram of another example I/O circuit 700 in accordance with some embodiments. Components that are the same or similar to those depicted in FIG. 1 are given the same reference numbers, and detailed description thereof is thus omitted.

Compared with I/O circuit 100, circuit 700 includes K P-type cascode driver transistors 720[1], 720[2], 720[3], . . . , 720[K] coupled in series between power node 112 and output node 104; K N-type cascode driver transistors 730[1], 730[2], 730[3], . . . , 730[K] coupled in series between power node 114 and output node 104; and a biasing circuit 740 replacing biasing circuit 140. Output signal V_(OUT) has a voltage level ranging from the zero reference level to a predetermined voltage level K·V_(DD). Power node 112 is configured to receive a voltage having a voltage level of K·V_(DD). Power node 116 is configured to receive a voltage having a voltage level of (K−1)·V_(DD). K is a positive ratio greater than 1. In some embodiments when K is set to be 2, circuit 700 becomes the same as I/O circuit 100, where driver transistor 720[1] corresponds to driver transistor 122; driver transistor 720[2] corresponds to driver transistor 124; driver transistor 730[1] corresponds to driver transistor 132; and driver transistor 730[2] corresponds to driver transistor 134.

Biasing circuit 740 is coupled with input node 102 and gates of driver transistors 720[1], 720[2], 720[3], . . . , 720[K], 730[1], 730[2], 730[3], . . . , and 730[K]. Biasing circuit 740 is configured to turn off driver transistors 720 and turn on driver transistors 730 responsive to input signal V_(IN) being at the logical high value; and to turn on driver transistors 720 and turn off driver transistors 730 responsive to input signal V_(IN) being at the logical low value. The operation of contending circuits 150 and 160 in I/O circuit 700 are similar to the operation of contending circuits 150 and 160 in I/O circuit 100, and detailed description is thus omitted.

FIG. 8 is a flow chart of a method 800 of operating an I/O circuit in accordance with some embodiments. To facilitate the explanation of method 800, I/O circuit 100 is referred to as an example. However, method 800 is also applicable to I/O circuit 600 or 700. It is understood that additional operations may be performed before, during, and/or after the method 800 depicted in FIG. 8, and that some other processes may only be briefly described herein.

As depicted in FIG. 8 and FIG. 1, the method 800 begins with operation 810, where an output node is electrically coupled with a power node through a driver transistor responsive to an edge of an input signal. For example, if input signal V_(IN) transitions from the zero reference level to V_(DD), output node 104 is electrically coupled with power node 114 through driver transistor 132. Also, if input signal V_(IN) transitions from V_(DD) to the zero reference level, output node 104 is electrically coupled with power node 112 through driver transistor 122.

The method 800 proceeds to operation 820, where a control signal V_(CDP) or V_(CDN) is generated based on a signal V_(INP) or V_(INN) at a gate of the driver transistor 122 or 132 and a slew rate of a signal V_(OUT) at the output node 104.

Operation 820 includes operation 822 and operation 824. In operation 822, a falling edge of signal V_(CDP) is generated by delaying a corresponding falling edge of signal V_(INP) for a delay period T_(D2), and the delay period T_(D2) is determined based on the slew rate of a rising edge of signal V_(OUT). Also, in operation 822, a rising edge of signal V_(CDN) is generated by delaying a corresponding rising edge of signal V_(INN) for a delay period T_(D1), and the delay period T_(D1) is determined based on the slew rate of a falling edge of signal V_(OUT).

In operation 824, when control circuit 400 in FIG. 4 is being used, a rising edge of signal V_(CDP) is generated by passing through a corresponding rising edge of signal V_(INP), and thus avoiding further delaying rising edge of signal V_(CDP) as compared with the corresponding rising edge of signal V_(INP). Also, in operation 824, when control circuit 300 in FIG. 3 is being used, a falling edge of signal V_(CDN) is generated by passing through a corresponding falling edge of signal V_(INN), and thus avoiding further delaying falling edge of signal V_(CDN) as compared with the corresponding falling edge of signal V_(INN).

In some embodiments when control circuit 500 in FIG. 5 is being used instead of control circuit 300 and control circuit 400, operation 824 is omitted.

The method 800 proceeds to operation 830, where a contending circuit is caused to contend with the driver transistor by electrically coupling a drain of the driver transistor with another power node through the contending circuit responsive to the control signal. For example, when driver transistor 122 is turned on to pull signal V_(DP) toward K·V_(DD) at power node 112, control signal V_(CDP) causes contending transistor 156 to pull signal V_(DP) toward (K−1)·V_(DD) during the delay period T_(D2). Also, when driver transistor 132 turned on to pull signal V_(DN) toward the zero reference level at power node 114, control signal V_(CDN) causes contending transistor 166 to pull signal V_(DN) toward V_(DD) during the delay period T_(D1).

In some embodiments, a circuit includes a first power node configured to carry a first voltage having a first voltage level, a second power node configured to carry a second voltage having a second voltage level, an output node, first and second cascode transistors coupled between the first power node and the output node and to each other at a node, a bias circuit configured to use the first and second cascode transistors to generate an output signal at the output node that transitions between the first voltage level and a third voltage level, a delay circuit configured to generate a transition in a first signal from one of the first or second voltage levels to the other of the first or second voltage levels, the transition having a time delay based on the output signal, and a contending transistor configured to, responsive to the first signal, couple the node to the second power node. In some embodiments, the delay circuit is configured to increase the time delay responsive to a decrease in a slew rate of the output signal, thereby increasing a duration of the node being coupled to the second power node, and decrease the time delay responsive to an increase in the slew rate of the output signal, thereby decreasing the duration of the node being coupled to the second power node. In some embodiments, the circuit is configured to control the time delay to cause a voltage level on the node to differ from a voltage level of the output signal by a magnitude equal to or less than a magnitude of a difference between the first and second voltage levels. In some embodiments, magnitude of the difference between the first and second voltage levels corresponds to a hot-carrier-injection over-stress level of a source-to-drain voltage of the first and second cascode transistors. In some embodiments, the circuit includes a logic gate configured to receive the first signal and generate a control signal, each of the delay circuit and the logic gate is configured to receive a second signal from the bias circuit, the delay circuit is configured to generate the transition in the first signal responsive to a first transition in the second signal, the logic gate is configured to generate a first transition in the control signal responsive to the transition in the first signal and a second transition in the control signal responsive to a second transition in the second signal, and the contending transistor is configured to receive the control signal, decouple the node from the second power node responsive to the first transition in the control signal, and couple the node to the second power node responsive to the second transition in the control signal. In some embodiments, the one of the first or second cascode transistors coupled between the first power node and the node is one of a P-type or N-type transistor having a first driving capability, and the contending transistor is the other of the P-type or N-type transistor having a second driving capability equal to or less than the first driving capability. In some embodiments, the bias circuit is configured to receive an input signal and use the first and second cascode transistors to generate the output signal responsive to the input signal.

In some embodiments, a circuit includes a first power node configured to carry a first voltage having a first voltage level, a second power node configured to carry a second voltage having a second voltage level, a third power node configured to carry a third voltage having a third voltage level, a fourth power node configured to carry a fourth voltage having a zero reference level, an output node, first and second cascode transistors coupled between the first power node and the output node and to each other at a first node, third and fourth cascode transistors coupled between the output node and the fourth power node and to each other at a second node, a bias circuit configured to use the first through fourth cascode transistors to generate an output signal at the output node that transitions between the first voltage level and the zero reference level, a first delay circuit configured to generate a falling edge in a first signal from the first voltage level to the second voltage level, the falling edge having a first time delay based on the output signal, a first contending transistor configured to, responsive to the first signal, couple the first node to the second power node, a second delay circuit configured to generate a rising edge in a second signal from the zero reference level to the third voltage level, the rising edge having a second time delay based on the output signal, and a second contending transistor configured to, responsive to the second signal, couple the second node to the third power node. In some embodiments, the first delay circuit is configured to increase the first time delay responsive to a decrease in a slew rate of a rising edge of the output signal, thereby increasing a duration of the first node being coupled to the second power node, and decrease the first time delay responsive to an increase in the slew rate of the rising edge of the output signal, thereby decreasing the duration of the first node being coupled to the second power node, and the second delay circuit is configured to increase the second time delay responsive to a decrease in a slew rate of a falling edge of the output signal, thereby increasing a duration of the second node being coupled to the third power node, and decrease the second time delay responsive to an increase in the slew rate of the falling edge of the output signal, thereby decreasing the duration of the second node being coupled to the third power node. In some embodiments, the circuit is configured to control the first time delay to cause a voltage level on the first node to differ from a voltage level of the output signal by a magnitude equal to or less than a magnitude of a difference between the third voltage level and the zero reference level, and control the second time delay to cause a voltage level on the second node to differ from the voltage level of the output signal by a magnitude equal to or less than the magnitude of the difference between the third voltage level and the zero reference level. In some embodiments, the magnitude of the difference between the third voltage level and the zero reference level corresponds to a hot-carrier-injection over-stress level of a source-to-drain voltage of the first through fourth cascode transistors. In some embodiments, the circuit includes an OR gate configured to receive the first signal and generate a first control signal, wherein each of the first delay circuit and the OR gate is configured to receive a third signal from the bias circuit, the first delay circuit is configured to generate the falling edge in the first signal responsive to a falling edge in the third signal, the OR gate is configured to generate a falling edge in the first control signal responsive to the falling edge in the first signal, and a rising edge in the first control signal responsive to a rising edge in the third signal, and the first contending transistor is configured to receive the first control signal, decouple the first node from the second power node responsive to the falling edge in the first control signal, and couple the first node to the second power node responsive to the rising edge in the first control signal, and an AND gate configured to receive the second signal and generate a second control signal, wherein each of the second delay circuit and the AND gate is configured to receive a fourth signal from the bias circuit, the second delay circuit is configured to generate the rising edge in the second responsive to a rising edge in the fourth signal, the AND gate is configured to generate a rising edge in the second control signal responsive to the rising edge in the first signal, and a falling edge in the second control signal responsive to a falling edge in the fourth signal, and the second contending transistor is configured to receive the second control signal, decouple the second node from the third power node responsive to the rising edge in the second control signal, and couple the second node to the third power node responsive to the falling edge in the second control signal. In some embodiments, the one of the first or second cascode transistors coupled between the first power node and the first node is a P-type transistor having a first driving capability, and the first contending transistor is an N-type transistor having a second driving capability equal to or less than the first driving capability, the one of the third or fourth cascode transistors coupled between the fourth power node and the second node is an N-type transistor having a third driving capability, and the second contending transistor is a P-type transistor having a fourth driving capability equal to or less than the third driving capability. In some embodiments, the bias circuit is configured to receive an input signal and use the first through fourth cascode transistors to generate the output signal responsive to the input signal.

In some embodiments, a method includes using a bias circuit to control first and second cascode transistors coupled between a first power node and an output node of a circuit, thereby generating an output signal at the output node, the output signal ranging between a first voltage level carried on the first power node and a second voltage level, using a contending transistor to selectively couple a node between the first and second cascode transistors to a second power node carrying a third voltage level, and based on the output signal, using a delay circuit to adjust a time delay of a transition in a first signal from one of the first or third voltage levels to the other of the first or third voltage levels, wherein the selective coupling of the contending transistor is based on the adjusted time delay. In some embodiments, the using the delay circuit to adjust the time delay includes increasing the time delay in response to a decrease in a slew rate of the output signal, thereby increasing a duration of the node being coupled to the second power node, and decreasing the time delay in response to an increase in the slew rate of the output signal, thereby decreasing the duration of the node being coupled to the second power node. In some embodiments, using the delay circuit to adjust the time delay includes controlling the time delay to maintain a difference between a voltage level on the node and a voltage level of the output signal at a magnitude equal to or less than a magnitude of a difference between the first and second voltage levels. In some embodiments, maintaining the difference between the first and second voltage levels at the magnitude corresponds to maintaining a source-to-drain voltage of one of the first or second cascode transistors below a hot-carrier-injection over-stress level. In some embodiments, the method includes receiving the first signal at a logic gate, receiving a second signal from the bias circuit at the logic gate and at the delay circuit, and using the logic gate to generate a control signal in response to the first and second signals, using the logic gate to generate the control signal including generating a first transition in the control signal in response to the transition in the first signal, and generating a second transition in the control signal in response to a transition in the second signal, wherein using the delay circuit to adjust the time delay of the transition in the first signal is in response to the transition in the second signal, and using the contending transistor to selectively couple the node comprises decoupling the node from the second power node in response to the first transition in the control signal, and coupling the node to the second power node in response to the second transition in the control signal. In some embodiments, one of the first or second cascode transistors is coupled between the first power node and the node and is one of a P-type or N-type transistor having a first driving capability, and using the contending transistor to selectively couple the node includes using the other of the P-type or N-type transistor having a second driving capability equal to or less than the first driving capability.

Various types of transistors are discussed in this disclosure as example. In some embodiments, the implementations using other types of transistors different from those illustrated in the present disclosure are within the scope of the subject application.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A circuit comprising: a first power node configured to carry a first voltage having a first voltage level; a second power node configured to carry a second voltage having a second voltage level; an output node; first and second cascode transistors coupled between the first power node and the output node and to each other at a node; a bias circuit configured to use the first and second cascode transistors to generate an output signal at the output node that transitions between the first voltage level and a third voltage level; a delay circuit configured to generate a transition in a first signal from one of the first or second voltage levels to the other of the first or second voltage levels, the transition having a time delay based on the output signal; and a contending transistor configured to, responsive to the first signal, couple the node to the second power node.
 2. The circuit of claim 1, wherein the delay circuit is configured to increase the time delay responsive to a decrease in a slew rate of the output signal, thereby increasing a duration of the node being coupled to the second power node, and decrease the time delay responsive to an increase in the slew rate of the output signal, thereby decreasing the duration of the node being coupled to the second power node.
 3. The circuit of claim 1, wherein the circuit is configured to control the time delay to cause a voltage level on the node to differ from a voltage level of the output signal by a magnitude equal to or less than a magnitude of a difference between the first and second voltage levels.
 4. The circuit of claim 3, wherein the magnitude of the difference between the first and second voltage levels corresponds to a hot-carrier-injection over-stress level of a source-to-drain voltage of the first and second cascode transistors.
 5. The circuit of claim 1, further comprising a logic gate configured to receive the first signal and generate a control signal, wherein each of the delay circuit and the logic gate is configured to receive a second signal from the bias circuit, the delay circuit is configured to generate the transition in the first signal responsive to a first transition in the second signal, the logic gate is configured to generate a first transition in the control signal responsive to the transition in the first signal, and a second transition in the control signal responsive to a second transition in the second signal, and the contending transistor is configured to receive the control signal, decouple the node from the second power node responsive to the first transition in the control signal, and couple the node to the second power node responsive to the second transition in the control signal.
 6. The circuit of claim 1, wherein the one of the first or second cascode transistors coupled between the first power node and the node is one of a P-type or N-type transistor having a first driving capability, and the contending transistor is the other of the P-type or N-type transistor having a second driving capability equal to or less than the first driving capability.
 7. The circuit of claim 1, wherein the bias circuit is configured to receive an input signal and use the first and second cascode transistors to generate the output signal responsive to the input signal.
 8. A circuit comprising: a first power node configured to carry a first voltage having a first voltage level; a second power node configured to carry a second voltage having a second voltage level; a third power node configured to carry a third voltage having a third voltage level; a fourth power node configured to carry a fourth voltage having a zero reference level; an output node; first and second cascode transistors coupled between the first power node and the output node and to each other at a first node; third and fourth cascode transistors coupled between the output node and the fourth power node and to each other at a second node; a bias circuit configured to use the first through fourth cascode transistors to generate an output signal at the output node that transitions between the first voltage level and the zero reference level; a first delay circuit configured to generate a falling edge in a first signal from the first voltage level to the second voltage level, the falling edge having a first time delay based on the output signal; a first contending transistor configured to, responsive to the first signal, couple the first node to the second power node; a second delay circuit configured to generate a rising edge in a second signal from the zero reference level to the third voltage level, the rising edge having a second time delay based on the output signal; and a second contending transistor configured to, responsive to the second signal, couple the second node to the third power node.
 9. The circuit of claim 8, wherein the first delay circuit is configured to increase the first time delay responsive to a decrease in a slew rate of a rising edge of the output signal, thereby increasing a duration of the first node being coupled to the second power node, and decrease the first time delay responsive to an increase in the slew rate of the rising edge of the output signal, thereby decreasing the duration of the first node being coupled to the second power node, and the second delay circuit is configured to increase the second time delay responsive to a decrease in a slew rate of a falling edge of the output signal, thereby increasing a duration of the second node being coupled to the third power node, and decrease the second time delay responsive to an increase in the slew rate of the falling edge of the output signal, thereby decreasing the duration of the second node being coupled to the third power node.
 10. The circuit of claim 8, wherein the circuit is configured to control the first time delay to cause a voltage level on the first node to differ from a voltage level of the output signal by a magnitude equal to or less than a magnitude of a difference between the third voltage level and the zero reference level, and control the second time delay to cause a voltage level on the second node to differ from the voltage level of the output signal by a magnitude equal to or less than the magnitude of the difference between the third voltage level and the zero reference level.
 11. The circuit of claim 10, wherein the magnitude of the difference between the third voltage level and the zero reference level corresponds to a hot-carrier-injection over-stress level of a source-to-drain voltage of the first through fourth cascode transistors.
 12. The circuit of claim 8, further comprising: an OR gate configured to receive the first signal and generate a first control signal, wherein each of the first delay circuit and the OR gate is configured to receive a third signal from the bias circuit, the first delay circuit is configured to generate the falling edge in the first signal responsive to a falling edge in the third signal, the OR gate is configured to generate a falling edge in the first control signal responsive to the falling edge in the first signal, and a rising edge in the first control signal responsive to a rising edge in the third signal, and the first contending transistor is configured to receive the first control signal, decouple the first node from the second power node responsive to the falling edge in the first control signal, and couple the first node to the second power node responsive to the rising edge in the first control signal; and an AND gate configured to receive the second signal and generate a second control signal, wherein each of the second delay circuit and the AND gate is configured to receive a fourth signal from the bias circuit, the second delay circuit is configured to generate the rising edge in the second responsive to a rising edge in the fourth signal, the AND gate is configured to generate a rising edge in the second control signal responsive to the rising edge in the first signal, and a falling edge in the second control signal responsive to a falling edge in the fourth signal, and the second contending transistor is configured to receive the second control signal, decouple the second node from the third power node responsive to the rising edge in the second control signal, and couple the second node to the third power node responsive to the falling edge in the second control signal.
 13. The circuit of claim 8, wherein the one of the first or second cascode transistors coupled between the first power node and the first node is a P-type transistor having a first driving capability, and the first contending transistor is an N-type transistor having a second driving capability equal to or less than the first driving capability, the one of the third or fourth cascode transistors coupled between the fourth power node and the second node is an N-type transistor having a third driving capability, and the second contending transistor is a P-type transistor having a fourth driving capability equal to or less than the third driving capability.
 14. The circuit of claim 8, wherein the bias circuit is configured to receive an input signal and use the first through fourth cascode transistors to generate the output signal responsive to the input signal.
 15. A method comprising: using a bias circuit to control first and second cascode transistors coupled between a first power node and an output node of a circuit, thereby generating an output signal at the output node, the output signal ranging between a first voltage level carried on the first power node and a second voltage level; using a contending transistor to selectively couple a node between the first and second cascode transistors to a second power node carrying a third voltage level; and based on the output signal, using a delay circuit to adjust a time delay of a transition in a first signal from one of the first or third voltage levels to the other of the first or third voltage levels, wherein the selective coupling of the contending transistor is based on the adjusted time delay.
 16. The method of claim 15, wherein the using the delay circuit to adjust the time delay comprises: increasing the time delay in response to a decrease in a slew rate of the output signal, thereby increasing a duration of the node being coupled to the second power node, and decreasing the time delay in response to an increase in the slew rate of the output signal, thereby decreasing the duration of the node being coupled to the second power node.
 17. The method of claim 15, wherein the using the delay circuit to adjust the time delay comprises controlling the time delay to maintain a difference between a voltage level on the node and a voltage level of the output signal at a magnitude equal to or less than a magnitude of a difference between the first and second voltage levels.
 18. The method of claim 17, wherein the maintaining the difference between the first and second voltage levels at the magnitude corresponds to maintaining a source-to-drain voltage of one of the first or second cascode transistors below a hot-carrier-injection over-stress level.
 19. The method of claim 15, further comprising: receiving the first signal at a logic gate; receiving a second signal from the bias circuit at the logic gate and at the delay circuit; and using the logic gate to generate a control signal in response to the first and second signals, the using the logic gate to generate the control signal comprising: generating a first transition in the control signal in response to the transition in the first signal; and generating a second transition in the control signal in response to a transition in the second signal, wherein the using the delay circuit to adjust the time delay of the transition in the first signal is in response to the transition in the second signal, and the using the contending transistor to selectively couple the node comprises decoupling the node from the second power node in response to the first transition in the control signal, and coupling the node to the second power node in response to the second transition in the control signal.
 20. The method of claim 15, wherein one of the first or second cascode transistors is coupled between the first power node and the node and is one of a P-type or N-type transistor having a first driving capability, and the using the contending transistor to selectively couple the node comprises using the other of the P-type or N-type transistor having a second driving capability equal to or less than the first driving capability. 